Zoom systems, i.e. variable magnification systems, are used today in almost every surgical microscope. One important representative thereof is the M690 (Leica brochure: Ophthalmologie [Ophthalmology] M1-602-0de-1.94-SCH, January 1994 printing).
The zoom system in this known microscope comprises an even number of optical elements. It has a four-element configuration and is constructed symmetrically. Each two lens groups are identical, and the groups are arranged in mirror-image fashion with respect to each other. The two outer lens groups are arranged immovably, the two inner groups movably. The two outer lens groups have positive refractive power, the two inner ones negative refractive power. Both focusing and a change in magnification are possible therewith.
The inventor has recognized that the systems existing today are capable of being improved in terms of chromatic aberrations, in particular those of the secondary system, and with regard to curvature of field and astigmatism. It is likewise desirable to keep the overall length short without having to sacrifice a high zoom factor.